Imaging lens and imaging apparatus

ABSTRACT

An imaging lens provided with a negative first lens group and a positive second lens group arranged in order from the object side. The first lens group is composed of a first group first lens which is a negative single lens, and the second lens group is composed of a positive second group first lens, a positive second group second lens, and a negative second group third lens arranged in order from the object side. The second group first lens is a biconvex lens, the second group second lens is a biconvex lens, the second group third lens is a meniscus lens, an aperture stop is disposed between the second group first and second lenses, and the second group second and third lenses form a cemented lens. The imaging lens satisfies conditional expressions (1): −0.89≦f1/f&lt;0 and (4): 0.3&lt;dt1/f&lt;0.8.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/JP2010/004677 filed on Jul.24, 2012, which claims foreign priority to Japanese Application No.2011-166421 filed on Jul. 29, 2011. The entire contents of each of theabove applications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to an imaging lens and animaging apparatus, and more specifically to an imaging lens that uses animage sensor, such as a CCD (Charge Coupled Device), a CMOS(Complementary Metal Oxide Semiconductor), or the like, and is used forsurveillance cameras, mobile terminal cameras, in-vehicle cameras, andthe like. The invention also relates to an imaging apparatus equippedwith the imaging lens.

BACKGROUND ART

Recently, as image sensors, such as CCDs, CMOSs, and the like, verysmall image sensors with increased pixel count have been known. Alongwith this, downsized imaging device bodies equipped with these imagesensors have also been known, and with respect to the imaging lenses foruse with these imaging device bodies, those downsized while maintainingfavorable optical performance are applied. In the mean time, in theapplications of surveillance cameras and in-vehicle cameras, thoseequipped with a small imaging lens yet having a wide angle of view andhigh performance have been known.

As imaging lenses having a wide angle of view with a relatively smallnumber of lenses known in the aforementioned fields, those described,for example, in Japanese Unexamined Patent Publication No. 9(1997)-281387, Japanese Unexamined Patent Publication No. 2(1990)-284108, Japanese Unexamined Patent Publication No. 2005-316208,and Japanese Unexamined Patent Publication No. 2011-128210 may be cited.

DISCLOSURE OF THE INVENTION

The imaging lens described in Japanese Unexamined Patent Publication No.9 (1997)-281387, however, is dark with an F-number of 2.8 and has largechromatic aberration and astigmatism, so that the imaging lens can notbe said to have so high optical performance as to be recommended for theapplication to such high pixel count and high performance image sensorsas described above.

Also, the imaging lens described in Japanese Unexamined PatentPublication No. 2 (1990)-284108 is dark with an F-number of 3.0 and haslarge chromatic aberration and astigmatism, so that that the imaginglens can not be said to have so high optical performance as to berecommended for the application to such high performance image sensorsas described above.

The imaging lens described in Japanese Unexamined Patent Publication No.2005-316208 is dark with an F-number of 2.8 and has large astigmatism,although chromatic aberration is well corrected. Therefore, as in theabove, it cannot be said that the imaging lens has so high opticalperformance as to be recommended for the application to such highperformance image sensors as described above.

The imaging lens described in Japanese Unexamined Patent Publication No.2011-128210 has tried to realize a bright lens but compactness issomewhat sacrificed for the sake of achieving the brightness and cannotbe said to be sufficiently downsized.

As such, in an imaging lens with a relatively small number of lenses,e.g., four lenses, there is a demand to use a high optical performanceimaging lens that satisfies both wide angle of view and compactness.More specifically, a wide angle and compact imaging lens, which is abright optical system with an F-number of about 2.0, well corrected inaberration is anticipated.

The present invention has been developed in view of the circumstancesdescribed above, and it is an object of the present invention to providea wide angle and compact imaging lens having high optical performance,and an imaging apparatus equipped with the imaging lens.

A first imaging lens of the present invention is an imaging lenssubstantially consisting of a first lens group having a negativerefractive power and a second lens group having a positive refractivepower, disposed in order from the object side, wherein: the first lensgroup is composed of a first group first lens which is a single lenshaving a negative refractive power; the second lens group is composed ofa second group first lens having a positive refractive power, a secondgroup second lens having a positive refractive power, and a second groupthird lens having a negative refractive power, disposed in order fromthe object side; and the imaging lens satisfies a conditional expression(1): −0.89≦f1/f<0, where f1 is the focal length of the first group firstlens at the d-line and f is the focal length of the entire lens systemat the d-line.

The imaging lens described above preferably satisfies a conditionalexpression (1a): −0.85≦f1/f<−0.3 and more preferably satisfies aconditional expression (1b): −0.72≦f1/f<−0.5.

A second imaging lens of the present invention is an imaging lenssubstantially consisting of a first lens group having a negativerefractive power and a second lens group having a positive refractivepower, disposed in order from the object side, wherein: the first lensgroup is composed of a first group first lens which is a biconcavesingle lens having a negative refractive power; the second lens group iscomposed of a second group first lens having a positive refractivepower, a second group second lens having a positive refractive power,and a second group third lens having a negative refractive power,disposed in order from the object side; and the imaging lens satisfies aconditional expression (2): 0≦dk2/f<0.7, where: dk2 is the distance (airequivalent distance) between the first group first lens and the secondgroup first lens on the optical axis; and f is the focal length of theentire lens system at the d-line. If no optical member is disposedbetween the first group first lens and the second group first lens, theaforementioned distance simply becomes air distance.

The imaging lens described above preferably satisfies a conditionalexpression (2a): 0.1≦dk2/f<0.65 and more preferably satisfies aconditional expression (2b): 0.15≦dk2/f<0.62.

A third imaging lens of the present invention is an imaging lenssubstantially consisting of a first lens group having a negativerefractive power and a second lens group having a positive refractivepower, disposed in order from the object side, wherein: the first lensgroup is composed of a first group first lens which is a biconcavesingle lens having a negative refractive power; second lens group iscomposed of a second group first lens having a positive refractivepower, an aperture stop, a second group second lens having a positiverefractive power, and a second group third lens having a negativerefractive power, disposed in order from the object side; the secondgroup second lens and the second group third lens are cemented togetherto form a cemented lens; and the imaging lens satisfies a conditionalexpression (3): 38<νd1<70, where νd1 is the Abbe number of the firstgroup first lens with reference to the d-line.

The imaging lens described above preferably satisfies a conditionalexpression (3a): 40<νd1<68 and more preferably satisfies a conditionalexpression (3b): 41<νd1<66.

Each of the first to the third imaging lenses may be an imaging lens inwhich an aperture stop is disposed in the second lens group.

Each of the first to the third imaging lenses may be an imaging lens inwhich the second group first lens is a biconvex lens; the second groupsecond lens is a biconvex lens; the second group third lens is ameniscus lens; and an aperture stop is disposed between the second groupfirst lens and the second group second lens and the second group secondlens and the second group third lens are cemented together to form acemented lens.

When dt1 is taken as the thickness of the first group first lens on theoptical axis, each of the first to the third imaging lenses preferablysatisfies a conditional expression (4): 0.3<dt1/f<0.8, more preferablysatisfies a conditional expression (4a): 0.31<dt1/f<0.6, and furtherpreferably satisfies a conditional expression (4b): 0.32<dt1/f<0.5.

When fg2 is taken as the combined focal length of the entire second lensgroup at the d-line, each of the first to the third imaging lensespreferably satisfies a conditional expression (5): 0<fg2/f<1.3, morepreferably satisfies a conditional expression (5a): 0.3<fg2/f<1.28, andfurther preferably satisfies a conditional expression (5b):0.5<fg2/f<1.25.

When νd2 is taken as the Abbe number of the second group first lens withreference to the d-line, each of the first to the third imaging lensespreferably satisfies a conditional expression (6): 35<νd2<70, morepreferably satisfies a conditional expression (6a): 38<νd2<68, andfurther preferably satisfies a conditional expression (6b): 40<νd2<66.

In a case where an aperture is provided, each of the first to the thirdimaging lenses preferably satisfies a conditional expression (7):13.5<dsi<22, more preferably satisfies a conditional expression (7a):13.8<dsi<20, and further preferably satisfies a conditional expression(7b): 14<dsi<18, where dsi is the distance between the aperture stop andthe image plane on the optical axis (the back focus portion is expressedin terms of air equivalent distance). That is, the “distance between theaperture stop and the image plane on the optical axis” is the distancebetween the apex of the image side surface of the second group thirdlens to the image plane (back focus) expressed in term of air equivalentdistance (air equivalent distance is applied to the thickness of anoptical element having no refractive power disposed between theaforementioned apex and the image plane). Note that actual length isused for the distance between the aperture stop and the apex of theimage side surface of the second group third lens.

An imaging apparatus of the present invention includes any of the firstto the third imaging lenses.

In each of the first to the third imaging lenses described above, thesecond group first lens constituting the second lens group is a singlelens.

Each of the first to the third imaging lenses described above includesno optical element having a power between the first lens group and thesecond lens group. That is, each of the first to the third imaginglenses is configured not to include an optical member having arefractive power between the first lens group and the second lens group.

The term “an imaging lens substantially consisting of n lens groups” asused herein refers to an imaging lens provided with a lens havingsubstantially no refractive power, an optical element other than a lens,such as an aperture stop, a cover glass, or the like, a lens flange, alens barrel, an image sensor, a mechanical component, such as a camerashake correction mechanism, and the like, in addition to the n lensgroups.

Each of the first to the third imaging lenses may include a lens grouphaving a refractive power disposed on the image side of the second lensgroup.

According to the first imaging lens and imaging apparatus equipped withthe same, a first lens group having a negative refractive power and asecond lens group having a positive refractive power are provided inorder from the object side, in which the first lens group is composed ofa first group first lens which is a single lens having a negativerefractive power, the second lens group is composed of a second groupfirst lens having a positive refractive power, a second group secondlens having a positive refractive power, and a second group third lenshaving a negative refractive power disposed in order from the objectside, and, when f1 is taken as the focal length of the first group firstlens and f is taken as the focal length of the entire lens system, theimaging lens is configured to satisfy a conditional expression (1):−0.89≦f1/f<0. This allows the first imaging lens and imaging apparatusequipped with the same to be compact with a wide angle of view and highoptical performance. For example, the first imaging lens may be a wideangle and compact imaging lens well corrected in aberration and brightwith an F-number of about 2.0.

According to the second imaging lens and imaging apparatus equipped withthe same, a first lens group having a negative refractive power and asecond lens group having a positive refractive power are provided inorder from the object side, in which the first lens group is composed ofa first group first lens which is a biconcave single lens having anegative refractive power, the second lens group is composed of a secondgroup first lens having a positive refractive power, a second groupsecond lens having a positive refractive power, and a second group thirdlens having a negative refractive power, disposed in order from theobject side, and, when dk2 is taken as the distance between the firstgroup first lens and the second group first lens on the optical axis,the imaging lens is configured to satisfy a conditional expression (2):0dk2/f<0.70. This allows the second imaging lens and imaging apparatusequipped with the same to be compact with a wide angle of view and highoptical performance. For example, the second imaging lens may be a wideangle and compact imaging lens well corrected in aberration and brightwith an F-number of about 2.0.

According to the third imaging lens and imaging apparatus equipped withthe same, a first lens group having a negative refractive power and asecond lens group having a positive refractive power are provided inorder from the object side, in which the first lens group is composed ofa first group first lens which is a biconcave single lens having anegative refractive power, the second lens group is composed of a secondgroup first lens having a positive refractive power, an aperture stop, asecond group second lens having a positive refractive power, and asecond group third lens having a negative refractive power, disposed inorder from the object side, the second group second lens and the secondgroup third lens are cemented together to form a cemented lens, and,when νd1 is taken as the Abbe number of the first group first lens withreference to the d-line, the imaging lens is configured to satisfy aconditional expression (3): 38<νd1<70. This allows the third imaginglens and imaging apparatus equipped with the same to be compact with awide angle of view and high optical performance. For example, the thirdimaging lens may be a wide angle and compact imaging lens well correctedin aberration and bright with an F-number of about 2.0.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view of an imaging lens and imagingapparatus of a first embodiment of the present invention.

FIG. 1B is a cross-sectional view of an imaging lens and imagingapparatus of a second embodiment of the present invention.

FIG. 1C is a cross-sectional view of an imaging lens and imagingapparatus of a third embodiment of the present invention.

FIG. 2 illustrates a configuration of an imaging lens according toExample 1 with optical paths.

FIG. 3 illustrates a configuration of an imaging lens according toExample 2.

FIG. 4 illustrates a configuration of an imaging lens according toExample 3.

FIG. 5 illustrates a configuration of an imaging lens according toExample 4.

FIG. 6 illustrates a configuration of an imaging lens according toExample 5.

FIG. 7 shows in a to d aberration diagrams of the imaging lens accordingto Example 1.

FIG. 8 shows in a to d aberration diagrams of the imaging lens accordingto Example 2.

FIG. 9 shows in a to d aberration diagrams of the imaging lens accordingto Example 3.

FIG. 10 shows in a to d aberration diagrams of the imaging lensaccording to Example 4.

FIG. 11 shows in a to d aberration diagrams of the imaging lensaccording to Example 5.

FIG. 12 illustrates a surveillance camera equipped with the imaging lensof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings.

FIG. 1A is a cross-sectional view of an imaging lens and imagingapparatus according to a first embodiment of the present invention,illustrating the configuration thereof, FIG. 1B is a cross-sectionalview of an imaging lens and imaging apparatus according to a secondembodiment of the present invention, illustrating the configurationthereof, and FIG. 1C is a cross-sectional view of an imaging lens andimaging apparatus according to a third embodiment of the presentinvention, illustrating the configuration thereof.

As illustrated in FIG. 1A, an imaging apparatus 201 according to thefirst embodiment of the present invention includes an image sensor 210and an imaging lens 101 according to the first embodiment of the presentinvention. The image sensor 210 converts an optical image Imrepresenting a subject 1 formed on a light receiving surface 210J of theimage sensor 210 through the imaging lens 101 to an electrical signaland generates an image signal Gs representing the subject 1. As for theimage sensor 210, for example, a CCD image sensor, a CMOS image sensor,a MOS image sensor, or the like may be employed.

The imaging lens 101 includes a first lens group G1 having a negativerefractive power and a second lens group G2 having a positive refractivepower arranged in order from the object side (arrow −Z direction side inthe drawing). Note that no optical member having a power is disposedbetween the first lens group G1 and the second lens group G2.

The first lens group G1 includes only one lens of a first group firstlens L11 which is a single lens having a negative refractive power, asoptical member having a power.

The second lens group G2 includes a second group first lens L21 which isa single lens having a positive refractive power, a second group secondlens L22 having a positive refractive power, and a second group thirdlens L23 having a negative refractive power arranged in order from theobject side, as optical member having a power.

Further, the imaging lens 101 satisfies a conditional expression (1):−0.89≦f1/f<0, where f1 is the focal length of the first group first lensL11 and f is the focal length of the entire lens system.

Still further, the imaging lens 101 more preferably satisfies aconditional expression (1a): −0.85≦f1/f<−0.3 and further preferablysatisfies a conditional expression (1b): −0.72≦f1/f<−0.5.

As illustrated in FIG. 1B, an imaging apparatus 202 according to thesecond embodiment of the present invention includes an image sensor 210and an imaging lens 102 according to the second embodiment of thepresent invention. The structure and operation of the image sensor 210are identical to those in the imaging apparatus 201 described above.

The imaging lens 102 includes a first lens group G1 having a negativerefractive power and a second lens group G2 having a positive refractivepower arranged in order from the object side (arrow −Z direction side inthe drawing). Note that no optical member having a power is disposedbetween the first lens group G1 and the second lens group G2.

The first lens group G1 includes only one lens of a first group firstlens L11 which is a single lens having a negative refractive power asoptical member having a power.

The second lens group G2 includes a second group first lens L21 which isa single lens having a positive refractive power, a second group secondlens L22 having a positive refractive power, and a second group thirdlens L23 having a negative refractive power arranged in order from theobject side, as optical member having a power.

Further, the imaging lens 102 satisfies a conditional expression (2):0≦dk2/f<0.7, where dk2 is the distance (air equivalent distance) betweenthe first group first lens L11 and the second group first lens L21 onthe optical axis. If no optical member is disposed between the firstgroup first lens L11 and the second group first lens L21, the distancesimply becomes air distance.

Still further, the imaging lens 102 more preferably satisfies aconditional expression (2a): 0.1≦dk2/f<0.65 and further preferablysatisfies a conditional expression (2b): 0.15≦dk2/f<0.62.

As illustrated in FIG. 1C, an imaging apparatus 203 according to thethird embodiment of the present invention includes an image sensor 210and an imaging lens 103 according to the third embodiment of the presentinvention. The structure and operation of the image sensor 210 areidentical to those in the imaging apparatus 201 described above.

The imaging lens 103 includes a first lens group G1 having a negativerefractive power and a second lens group G2 having a positive refractivepower arranged in order from the object side (arrow −Z direction side inthe drawing). Note that no optical member having a power is disposedbetween the first lens group G1 and the second lens group G2.

The first lens group G1 includes only one lens of a first group firstlens L11 which is a biconcave single lens having a negative refractivepower, as optical member having a power.

The second lens group G2 includes a second group first lens L21 which isa single lens having a positive refractive power, a second group secondlens L22 having a positive refractive power, and a second group thirdlens L23 having a negative refractive power arranged in this order fromthe object side, as optical member having a power.

Note that an aperture stop St is disposed between the second group firstlens L21 and the second group second lens L22.

Further, the imaging lens 103 satisfies a conditional expression (3):38<νd1<70, where νd1 is the Abbe number of the first group first lensL11 with reference to the d-line.

Still further, the imaging lens 103 more preferably satisfies aconditional expression (3a): 40<νd1<68 and further preferably satisfiesa conditional expression (3b): 41<νd1<66.

Each of the imaging lens 101 of the first embodiment, the imaging lens102 of the second embodiment, and the imaging lens 103 of the thirdembodiment may also have the following configurations.

That is, each of the imaging lenses 101, 102 may include an aperturestop disposed in the second lens group G2 and, for example, the aperturestop St may be disposed between the second group first lens L21 and thesecond group second lens L22, as illustrated in FIGS. 1A, 1B.

Each of the imaging lens 101, 102 may have a configuration in which thesecond group first lens L21 is a biconvex lens, the second group secondlens L22 is a biconvex lens, the second group third lens L23 is ameniscus lens, an aperture stop is disposed between the second groupfirst lens L21 and the second group second lens L22, and the secondgroup second lens L22 and the second group third lens L23 are cementedtogether to form a cemented lens.

Each of the imaging lenses 101, 102, 103 preferably satisfies aconditional expression (4): 0.3<dt1/f<0.8, more preferably satisfies aconditional expression (4a): 0.31<dt1/f<0.6, and further preferablysatisfies a conditional expression (4b): 0.32<dt1/f<0.5, where dt1 isthe thickness of the first group first lens L11 on the optical axis.

Further, each of the imaging lenses 101, 102, 103 preferably satisfies aconditional expression (5): 0<fg2/f<1.3, more preferably satisfies aconditional expression (5a): 0.3<fg2/f<1.28, and further preferablysatisfies a conditional expression (5b): 0.5<fg2/f<1.25, where fg2 isthe combined focal length of the second lens group G2 (combined focallength of the entire second lens group G2).

Still further, each of the imaging lenses 101, 102, 103 preferablysatisfies a conditional expression (6): 35<νd2<70, more preferablysatisfies a conditional expression (6a): 38<νd2<68, and furtherpreferably satisfies a conditional expression (6b): 40<νd2<66, where νd2is the Abbe number of the second group first lens L21 with reference tothe d-line.

Further, each of the imaging lenses 101, 102, 103 preferably satisfies aconditional expression (7): 13.5<dsi<22, more preferably satisfies aconditional expression (7a): 13.8<dsi<20, and further preferablysatisfies a conditional expression (7b): 14<dsi<18, where dsi is thedistance between the aperture stop St and the image plane Im on theoptical axis (the back focus portion is expressed in terms of airequivalent distance). That is, the “distance between the aperture stopSt and the image plane Im on the optical axis” is the distance betweenthe apex of the image side surface of the second group third lens L23and the image plane Im (back focus) expressed in terms of air equivalentdistance by applying an air equivalent distance to the thickness of theoptical element LL, such as a cover glass or the like.

The effects of the conditional expressions (1), (2), (3), (4), (5), (6),(7) will now be described collectively.

[Effects of Conditional Expression (1): −0.89≦f1/f<0]

The conditional expression (1) defines the range of the ratio of the“focal length f1 of the first lens group G1 (first group first lensL11)” to the “focal length f of the entire lens system”.

By configuring the imaging lens to satisfy the conditional expression(1), the balance between spherical aberration and image plane aberrationmay be maintained in a favorable condition while achieving downsizing.

If the imaging lens is configured to exceed the upper limit of theconditional expression (1), the negative refractive power of the firstlens group G1 (first group first lens L11) is increased and sphericalaberration is increased, whereby the difference between the “sphericalaberration of the marginal rays” and the “spherical aberration of therays passing through the ray height corresponding to 70% of that of themarginal rays” is increased. Further, the peripheral sagittal imagesurface is inclined to the over side.

If the imaging lens is configured to fall below the lower limit of theconditional expression (1), the negative refractive power of the firstlens group G1 is reduced and the image plane is inclined to the underside. Further, the need to extend the overall lens length is increasedso that downsizing becomes difficult.

The effects of the conditional expressions (1a), (1b) are identical tothose of the conditional expression (1).

[Effects of Conditional Expression (2): 0≦dk2/f<0.7]

The conditional expression (2) defines the range of the ratio of the“distance dk2 (air equivalent distance) between the first lens group G1and the second lens group G2” to the “focal length f of the entire lenssystem”.

By configuring the imaging lens to satisfy the conditional expression(2), the balance between spherical aberration and image plane aberrationmay be maintained in a favorable condition while achieving downsizing.

If the imaging lens exceeds the upper limit of the conditionalexpression (2), the need to extend the overall lens length is increasedand downsizing becomes difficult.

If the imaging lens exceeds the upper limit of the conditionalexpression (2), the need to extend the overall lens length foraberration correction is increased and downsizing becomes difficult.

If the imaging lens falls below the lower limit of the conditionalexpression (2), problems arise that spherical aberration further tendsto be increased and the tangential image plane is inclined to the overside, although it is convenient for downsizing.

The effects of the conditional expressions (2a), (2b) are identical tothose of the conditional expression (2).

[Effects of Conditional Expression (3): 38<νd1<70]

The conditional expression (3) defines the range of the Abbe number ofthe first group first lens L11 disposed on the most object side.

By configuring the imaging lens to satisfy the conditional expression(3), chromatic aberration may be inhibited.

If the imaging lens exceeds the upper limit of the conditionalexpression (3), longitudinal chromatic aberration further tends to beunder-corrected and lateral chromatic aberration further tends to beover-corrected in the short wavelength side.

If the imaging lens falls below the lower limit of the conditionalexpression (3), longitudinal chromatic aberration further tends to beover-corrected and lateral chromatic aberration further tends to beunder-corrected in the short wavelength side.

The effects of the conditional expressions (3a), (3b) are identical tothose of the conditional expression (3).

[Effects of Conditional Expression (4): 0.3<dt1/f<0.8]

The conditional expression (4) defines the range of the ratio of the“thickness dt of the first group first lens L11 disposed on the mostobject side” to the “focal length f of the entire lens system”.

By configuring the imaging lens to satisfy the conditional expression(4), the overall lens length and the diameter of the imaging lens may bereduced, whereby downsizing may be achieved.

If the imaging lens exceeds the upper limit of the conditionalexpression (4), the variation in the spherical aberration to the changein the angle of view is increased. Further, the need to extend theoverall lens length for aberration correction is increased, so thatdownsizing becomes difficult.

If the imaging lens falls below the lower limit of the conditionalexpression (4), spherical aberration further tends to be increased,although it is convenient for downsizing the imaging lens.

The effects of the conditional expressions (4a), (4b) are identical tothose of the conditional expression (4).

[Effects of Conditional Expression (5): 0<fg2/f<1.3]

The conditional expression (5) defines the range of the ratio of the“combined focal length fg2 of the entire second lens group G2” to the“focal length f of the entire lens system”.

By configuring the imaging lens to satisfy the conditional expression(5), the balance between spherical aberration and image plane aberrationmay be maintained in a favorable condition while achieving downsizing ofthe imaging lens.

If the imaging lens exceeds the upper limit of the conditionalexpression (5), the balance in refractive power between the first lensgroup G1 and the subsequent group is disrupted, whereby the tangentialimage plane is inclined to the under side.

If the imaging lens falls below the lower limit of the conditionalexpression (5), the focal lengths of the first lens group G1 and thesubsequent group are both reduced and the refractive powers areincreased, so that high order spherical aberration is likely to occur.

The effects of the conditional expressions (5a), (5b) are identical tothose of the conditional expression (5).

[Effects of Conditional Expression (6): 35<νd2<70]

The conditional expression (6) defines the range of the Abbe number ofthe second group first lens L21 disposed on the most object side in thesecond lens group.

By configuring the imaging lens to satisfy the conditional expression(6), chromatic aberration that occurs when diffused light is convergedmay be inhibited.

If the imaging lens exceeds the upper limit of the conditionalexpression (6), longitudinal chromatic aberration further tends to beover-corrected in the short wavelength side.

If the imaging lens falls below the lower limit of the conditionalexpression (6), longitudinal chromatic aberration further tends to beunder-corrected in the short wavelength side.

The effects of the conditional expressions (6a), (6b) are identical tothose of the conditional expression (6).

[Effects of Conditional Expression (7): 13.5<dsi<22]

The conditional expression (7) defines the range of the aforementioned“distance between the aperture stop St and the image plane Im on theoptical axis (the back focus portion is expressed in terms of airequivalent distance)”.

By configuring the imaging lens to satisfy the conditional expression(7), the overall lens length and the diameter of the imaging lens may bereduced, whereby downsizing may be achieved.

If the imaging lens exceeds the upper limit of the conditionalexpression (7), the need to extend the overall lens length foraberration correction is increased and downsizing becomes difficult.Problems arise that the overall lens length needs to be extended inorder to obtain desired lens performance and lateral chromaticaberration is under corrected with respect to the light in the shortwavelength side.

On the other hand, if the imaging lens falls below the lower limit ofthe conditional expression (7), spherical aberration is increased andthe difference between the “spherical aberration of the marginal rays”and the “spherical aberration of the rays passing through the ray heightcorresponding to 70% of that of the marginal rays” is increased.

The effects of the conditional expressions (7a), (7b) are identical tothose of the conditional expression (7).

When applying each imaging lens described above to each imagingapparatus, optical elements LL having substantially no refractive power,such as a cover glass, a low-pass filter, an infrared cut filter, andthe like may be disposed between each of the imaging lenses 101 to 103and the image sensor 210 according to the structure of the imagingapparatus. For example, if each of the imaging lenses 101 to 103 ismounted on an in-vehicle camera and the camera is used as a nightsurveillance camera, it is preferable that a filter that cuts lighthaving wavelengths ranging from the ultraviolet to the blue light isinserted between the imaging lens and the image sensor.

Instead of disposing a low-pass filter and various types of filters thatcut specific wavelength regions between each of the imaging lenses 101to 103 and the image sensor 210, various types of filters may bedisposed between the lenses constituting the image lens or thin filmshaving identical effects to those of the various types of filters mayalso be formed (applying coatings) on the lens surfaces constituting theimaging lens.

If each of the imaging lenses 101 to 103 is used, for example, foroutdoor surveillance, the imaging lens is required to be usable in awide temperature range from the open air in a cold region to theinterior of a car in summer in a tropical region. In such a case, it ispreferable that the material of all of the lenses constituting eachimaging lens is glass. Further, all of the lenses constituting eachimaging lens are preferably spherical lenses in order to manufacture thelenses inexpensively. In a case where a priority is given to the opticalperformance over the cost, however, an aspherical lens may be employed.

As described above, the imaging lenses of the first to third embodimentsof the present invention have high optical performance and may realize awide angle of view and downsizing.

EXAMPLES

Examples that illustrate specific numerical data of the imaging lensaccording to the present invention will now be described.

Numerical data and the like of each of Examples 1 to 5 of the presentinvention will be described collectively with reference to FIGS. 2 to 6,FIGS. 7 to 11, and Tables 1 to 6. In FIGS. 2 to 6, reference symbolscorresponding to those in FIGS. 1A, 1B, and 1C representing the imaginglenses 101, 102, and 103 respectively indicate corresponding components.

Example 1

FIG. 2 illustrates a schematic configuration of the imaging lens ofExample 1 with optical paths of light passing through the imaging lens.

The imaging lens of Example 1 has a configuration corresponding to thoseof the imaging lenses of the first to third embodiments.

The imaging lens of Example 1 is configured to satisfy all of theconditional expressions (1), (2), (3), (4), (5), (6), (7).

Table 1 shows lens data of the imaging lens of Example 1. In the lensdata shown in Table 1, the surface number i represents i^(th) surface Siin which a number i (i=1, 2, 3, -----) is given to each surface in aserially increasing manner toward the image side with the most objectside surface being taken as the first surface. In the lens data shown inTable 1, the surface number is given also to an aperture stop St and anoptical element LL having no power.

The symbol Ri in Table 1 represents the radius of curvature of i^(th)(i=1, 2, 3, -----) surface and the symbol Di represents the surfacedistance between i^(th) surface and (i+1)^(th) surface on the opticalaxis Z1. The symbols Ri and Di correspond to the symbol Si (i=1, 2, 3,-----) in number.

The “dk2” in the conditional expression (2): 0<dk2/f<0.7 corresponds tothe surface distance represented by the symbol “D2” in the lens datadescribed above.

The “dt1” in the conditional expression (4): 0.3<dt1/f<0.8 correspondsto the surface distance (thickness of the lens) represented by thesymbol “D1” in the lens data described above.

The symbol Ndj represents the refractive index of j^(th) optical elementwith respect to the d-line (587.6 nm) in which a number j (j=1, 2, 3,-----) is given to each optical element in a serially increasing mannertoward the image side with the optical element on the most object sidebeing taken as the first optical element, and νdj represents the Abbenumber of j^(th) optical element with respect to the d-line. In Table 1,the unit of the radius of curvature and the surface distance is mm, andthe radius of curvature is positive if the surface is convex on theobject side and negative if it is convex on the image side.

Here, the first optical element corresponds to the first group firstlens L11, the second optical element corresponds to the second groupfirst lens L21, the third optical element corresponds to the secondgroup second lens L22, the fourth optical element corresponds to thesecond group third lens L23, and the fifth optical element correspondsthe optical element LL having no power. The optical element LL having nopower corresponds, for example, to a cover glass disposed on the lightreceiving surface of the image sensor or the like.

Because such optical systems as described above may generally maintainthe predetermined performance even when the sizes of the opticalelements, such as lenses and the like, are proportionally increased ordecreased, imaging lenses in which the entire lens data described aboveare proportionally increased or decreased may also be the examplesaccording to the present invention.

TABLE 1 Example 1 Lens Data Si Ri Di Ndj ν dj 1 −7.6230 2.50 1.83480742.7 2 8.3333 1.15 3 14.7208 6.50 1.834807 42.7 4 −9.9709 2.50 (St)5 ∞2.50 6 11.1582 4.00 1.729157 54.7 7 −5.8389 2.50 1.846660 23.8 8−23.2488 7.29 9 ∞ 2.41 1.516330 64.1 10 ∞

FIG. 7 illustrates spherical aberration, astigmatism, distortion, andlateral chromatic aberration of the imaging lens of Example 1. FIG. 7illustrates aberrations for the light of d-line, F-line, and C-line. Thediagram of astigmatism illustrates aberrations with respect to thesagittal image plane and the tangential image plane.

As illustrated in FIG. 7, the diagram indicated by the symbol arepresents the spherical aberration, the diagram indicated by the symbolb represents the astigmatism, the diagram indicated by the symbol crepresents the distortion, and the diagram indicated by the symbol drepresents the lateral chromatic aberration.

The diagram of the distortion illustrates an amount of displacement fromthe ideal image height f×tan θ, where f is the focal length of theentire lens system and θ is the half angle of view (treated as variable,0≦θ≦ω).

Table 6 at the end of the description of the examples shows the valuethat can be obtained by the numerical expression in each conditionalexpression with respect to each of Examples 1 to 5. The value of thenumerical expression in each conditional expression is the value withrespect to the d-line (wavelength 587.56 nm) and can be obtained fromthe lens data of the imaging lens shown in Table 1 and the like.

Note that the above descriptions of how to interpret FIG. 2 illustratingthe configuration of the imaging lens of Example 1, FIG. 7 illustratingthe aberrations of the imaging lens, Table 1 illustrating the lens dataof the imaging lens, correspondence between “dt1” in each conditionalexpression and “D1” in the lens data, correspondence between “dk2” ineach conditional expression and “D2” in the lens data, and Table 6showing the value of each numerical expression in each conditionalexpression apply to Examples 2 to 5 to be described later, so that thedescriptions thereof for the Examples that will follow are omitted.

Example 2

FIG. 3 illustrates a schematic configuration of the imaging lens ofExample 2. The imaging lens of Example 2 has a configurationcorresponding to those of the imaging lenses of the second and the thirdembodiments. But the imaging lens of Example 2 does not correspond tothe imaging lens of the first embodiment.

The imaging lens of Example 2 does not satisfy the conditionalexpression (1), but is configured to satisfy all the other conditionalexpressions (2), (3), (4), (5), (6), (7).

FIG. 8 illustrates aberrations of the imaging lens of Example 2.

Table 2 given below shows lens data of Example 2.

TABLE 2 Example 2 Lens Data Si Ri Di Ndj ν dj 1 −13.7732 2.50 1.51633064.1 2 3.9748 3.61 3 10.9651 6.50 1.743997 44.8 4 −8.7697 0.50 (St)5 ∞0.50 6 16.9378 2.99 1.620411 60.3 7 −4.5156 2.50 1.846660 23.8 8−17.3587 6.99 9 ∞ 2.41 1.516330 64.1 10 ∞

Example 3

FIG. 4 illustrates a schematic configuration of the imaging lens ofExample 3. The imaging lens of Example 3 has a configurationcorresponding to those of the imaging lenses of the second and the thirdembodiments. But the imaging lens of Example 3 does not correspond tothe imaging lens of the first embodiment.

The imaging lens of Example 3 does not satisfy the conditionalexpression (1), but is configured to satisfy all the other conditionalexpressions (2), (3), (4), (5), (6), (7).

FIG. 9 illustrates aberrations of the imaging lens of Example 3.

Table 3 given below shows lens data of Example 3.

TABLE 3 Example 3 Lens Data Si Ri Di Ndj ν dj 1 −12.4074 1.95 1.51633064.1 2 3.7700 2.86 3 11.9959 6.50 1.743997 44.8 4 −7.7766 0.60 (St)5 ∞0.50 6 18.1909 2.97 1.620411 60.3 7 −4.4533 2.50 1.808095 22.8 8−14.7223 6.65 9 ∞ 2.41 1.516330 64.1 10 ∞

Example 4

FIG. 5 illustrates a schematic configuration of the imaging lens ofExample 4. The imaging lens of Example 4 has a configurationcorresponding to those of the imaging lenses of the first to the thirdembodiments.

The imaging lens of Example 4 is configured to satisfy all of theconditional expressions (1), (2), (3), (4), (5), (6), (7).

FIG. 10 illustrates aberrations of the imaging lens of Example 4.

Table 4 given below shows lens data of Example 4.

TABLE 4 Example 4 Lens Data Si Ri Di Ndj ν dj 1 −7.6549 2.50 1.83480742.7 2 7.6549 1.15 3 13.5382 6.50 1.834807 42.7 4 −9.7830 2.50 (St)5 ∞2.50 6 10.9353 4.00 1.729157 54.7 7 −5.7927 2.50 1.846660 23.8 8−25.5605 7.19 9 ∞ 2.50 1.516330 64.1 10 ∞

Example 5

FIG. 6 illustrates a schematic configuration of the imaging lens ofExample 5. The imaging lens of Example 5 has a configurationcorresponding to those of the imaging lenses of the first to the thirdembodiments.

The imaging lens of Example 5 is configured to satisfy all of theconditional expressions (1), (2), (3), (4), (5), (6), (7).

FIG. 11 illustrates aberrations of the imaging lens of Example 5.

Table 5 given below shows lens data of Example 5.

TABLE 5 Example 5 Lens Data Si Ri Di Ndj ν dj 1 −13.2793 2.18 1.51633064.1 2 3.8297 2.43 3 14.8307 6.50 1.622992 58.2 4 −6.9328 2.50 (St)5 ∞0.50 6 15.6127 3.40 1.622992 58.2 7 −5.2992 1.54 1.808095 22.8 8−14.0503 7.47 9 ∞ 2.41 1.516330 64.1 10 ∞

Table 6 given below shows the value that can be obtained by thenumerical expression in each conditional expression, as described above.

TABLE 6 Condi- Numer- tional Ex- ical Ex- Exam- Exam- Exam- Exam- Exam-pression pression ple 1 ple 2 ple 3 ple 4 ple 5 (1) f1/f −0.69 *−0.92*−0.91 −0.67 −0.89 (2) dk2/f 0.18 0.58 0.48 0.18 0.39 (3) ν d1 42.7 64.164.1 42.7 64.1 (4) dt1/f 0.39 0.40 0.33 0.39 0.35 (5) fg2/f 1.23 1.121.13 1.21 1.20 (6) ν d2 42.7 44.8 44.8 42.7 58.2 (7) dsi 17.87 14.5614.19 17.76 14.49 *indicates value that does not satisfy conditionalexpression.

As can be seen from the foregoing, the imaging lenses of Examples 1 to 5have high optical performance and are compact imaging lenses having awide angle of view.

FIG. 12 illustrates a schematic configuration of a surveillance cameraas an embodiment of the imaging apparatus of the present invention. Thesurveillance camera 200 illustrated in FIG. 12 includes an imaging lens100 (e.g., imaging lens 101, 102, 103, or the like) disposed in asubstantially cylindrical lens barrel and an image sensor 210 thatcaptures an optical image of a subject formed by the imaging lens 100.The optical image formed on the light receiving surface of the imagesensor 210 through the imaging lens 100 is converted to an electricalsignal Gs and outputted from the surveillance camera 200.

So far the present invention has been described by way of the first tothe third embodiments and Examples, but the present invention is notlimited to the embodiments and Examples described above and variousmodifications can be made. For example, values of the radius ofcurvature of each lens element, surface distance, refractive index, Abbenumber, and the like are not limited to those shown in each NumericalExample and may take other values. For example, as a modification of theimaging lens having a cemented lens as shown in FIG. 1C, an imaging lensprovided with a lens group having a refractive power disposed on theimage side of the second lens group G2 may be cited.

In the embodiment of the imaging apparatus, the description andillustration have been made of a case in which the present invention isapplied to a surveillance camera. But the present invention is notlimited to such applications and is applicable, for example, to videocameras, electronic still cameras, in-vehicle cameras, and the like.

What is claimed is:
 1. An imaging lens substantially consisting of afirst lens group having a negative refractive power and a second lensgroup having a positive refractive power, disposed in order from theobject side, wherein: the first lens group is composed of a first groupfirst lens which is a single lens having a negative refractive power;the second lens group is composed of a second group first lens having apositive refractive power, a second group second lens having a positiverefractive power, and a second group third lens having a negativerefractive power, disposed in order from the object side; the secondgroup first lens is a biconvex lens; the second group second lens is abiconvex lens; the second group third lens is a meniscus lens; anaperture stop is disposed between the second group first lens and thesecond group second lens, and the second group second lens and thesecond group third lens are cemented together to form a cemented lens;and the imaging lens satisfies conditional expressions (1) and (4) givenbelow:−0.89≦f1/f<0  (1); and0.3<dt1/f<0.8  (4), where: f1: the focal length of the first group firstlens; f: the focal length of the entire lens system; and dt1: thethickness of the first group first lens on the optical axis.
 2. Theimaging lens of claim 1, wherein the imaging lens satisfies aconditional expression (1a) given below:−0.85≦f1/f<−0.3  (1a).
 3. The imaging lens of claim 1, wherein theimaging lens satisfies a conditional expression (1b) given below:−0.72≦f1/f<−0.5  (1b).
 4. The imaging lens of claim 1, wherein theimaging lens satisfies a conditional expression (4a) given below:0.31<dt1/f<0.6  (4a).
 5. The imaging lens of claim 4, wherein theimaging lens satisfies a conditional expression (4b) given below:0.32<dt1/f<0.5  (4b).
 6. The imaging lens of claim 1, wherein theimaging lens satisfies a conditional expression (5) given below:0<fg2/f<1.3  (5), where, fg2: the combined focal length of the entiresecond lens group.
 7. The imaging lens of claim 6, wherein the imaginglens satisfies a conditional expression (5a) given below:0.3<fg2/f<1.28  (5a).
 8. The imaging lens of claim 6, wherein theimaging lens satisfies a conditional expression (5b) given below:0.5<fg2/f<1.25  (5b).
 9. The imaging lens of claim 1, wherein theimaging lens satisfies a conditional expression (6) given below:35<νd2<70  (6), where, νd2: the Abbe number of the second group firstlens with reference to the d-line.
 10. The imaging lens of claim 9,wherein the imaging lens satisfies a conditional expression (6a) givenbelow:38<νd2<68  (6a).
 11. The imaging lens of claim 9, wherein the imaginglens satisfies a conditional expression (6b) given below:40<νd2<66  (6b).
 12. The imaging lens of claim 1, wherein the imaginglens satisfies a conditional expression (7) given below:13.5<dsi<22  (7), where, dsi: the distance between the aperture stop andthe image plane on the optical axis (the back focus portion is expressedin terms of air equivalent distance).
 13. The imaging lens of claim 12,wherein the imaging lens satisfies a conditional expression (7a) givenbelow:13.8<dsi<20  (7a).
 14. The imaging lens of claim 12, wherein the imaginglens satisfies a conditional expression (7b) given below:14<dsi<18  (7b).
 15. An imaging apparatus, comprising the imaging lensof claim
 1. 16. The imaging lens of claim 2, wherein the imaging lenssatisfies a conditional expression (4a) given below:0.31<dt1/f<0.6  (4a).
 17. The imaging lens of claim 16, wherein theimaging lens satisfies a conditional expression (4b) given below:0.32<dt1/f<0.5  (4b).
 18. The imaging lens of claim 2, wherein theimaging lens satisfies a conditional expression (5) given below:0<fg2/f<1.3  (5), where, fg2: the combined focal length of the entiresecond lens group.
 19. The imaging lens of claim 18, wherein the imaginglens satisfies a conditional expression (5a) given below:0.3<fg2/f<1.28  (5a).
 20. The imaging lens of claim 18, wherein theimaging lens satisfies a conditional expression (5b) given below:0.5<fg2/f<1.25  (5b).